1. Field of the Invention
The present invention relates to a fuel cell system suitable as a power source for an electronic apparatus or the like.
2. Description of the Background Art
In recent years, a great deal of attention has been paid to a fuel cell system as a power source for an electronic apparatus or the like capable of supplying electric power continuously for a long time. A fuel cell system is generally configured by an electrolyte layer, and an anode and a cathode sandwiching the electrolyte layer. It generates a direct current through an electrochemical reaction by supplying the anode with hydrogen and the cathode with oxygen. A fuel cell as a minimum unit generating such an electric current has a low electromotive voltage, thus requiring a series connection of a plurality of fuel cells to thereby obtain a voltage necessary for a fuel cell power apparatus. Hence, several fuel cells stacked and connected in series make up a fuel cell stack generating a higher output voltage and greater power.
A fuel cell system can produce a larger amount of electric current by enlarging the reaction area of an electrode. However, this enlargement makes the fuel cell system larger, thus unsuitable for a fuel cell system used in a portable electronic apparatus or the like requiring a smaller size. Therefore, in order to realize an active reaction on a massive scale in a limited reaction area, operation control is needed such as feeding a fuel smoothly to each fuel cell according to a target power for a load apparatus.
For the operation control, a method is known of supplying electric power to a load while storing the power generated by a fuel cell system in a secondary battery and regulates an input voltage or an input current into a DC/DC converter from a fuel cell based on feedback from the fuel cell (e.g., refer to Published Japanese Translation for PCT International Publication No. 2006-501798).
The above several fuel cells connected in series may differ in the output voltage of each fuel cell due to their characteristic dispersion when the supply of fuel to each fuel cell is uniform, and all the series-connected fuel cells have the same output current in the fuel cell stack including the series-connected fuel cells. If too much electric current flows through a fuel cell having a lower fuel stoichiometric ratio (=(supplied-fuel consuming rate)/(power-generating fuel consuming rate)) and a lower output voltage, the terminal voltage of the cell lower than any other cell is reversely polarized to the output voltage of the whole fuel cell stack, in other words, it becomes minus, called a fuel cell polarity reversal. This causes the problem of deteriorating the characteristics of a fuel cell.
Taking the problem into account, a method is known of controlling the supply of fuel or air to each fuel cell individually in a plurality of fuel cells connected in series to thereby reduce the output-voltage dispersion of each fuel cell (e.g., refer to Japanese Patent Laid-Open Publication No. 2006-73379).
Furthermore, a fuel-supply margin for a generated current becomes constant by keeping a fuel stoichiometric ratio (=supplied-fuel consuming rate/power-generating fuel consuming rate) constant. This makes it possible to control the fuel stoichiometric ratio constant at an appropriate value to thereby stabilize an output electric power. In addition to an output electric power, a power-generation efficiency which is the product of a voltage efficiency and a fuel utilization, is a crucial index for the performance of a power-generation portion of a fuel cell. The voltage efficiency is expressed by (fuel cell generated voltage/theoretical voltage), and the theoretical voltage is 1.21 V in a direct-methanol fuel cell. The fuel utilization is expressed by (power-generating fuel quantity/consumed fuel quantity).
A type of fuel cell supplying a water-soluble fuel and a solution thereof to an electrode, mainly including a direct-methanol fuel cell, has a so-called methanol-crossover problem of consuming a fuel (e.g., methanol) without generating energy by passing an electrolyte film supposed to isolate an anode from a cathode, thereby keeping a fuel utilization thereof commonly at 70 to 80%. This requires a fuel cell operation at an optimum fuel stoichiometric ratio to thereby maximize the output electric power and power-generation efficiency.
In view of fuel-supply and power-generation stability, therefore, it is desirable to generate power by operating a fuel cell system while controlling the fuel stoichiometric ratio constant. Accordingly, each fuel cell is generally supplied with a fixed quantity of fuel at a fixed concentration to thereby generate power at a constant current. For example, Published Japanese Translation for PCT International Publication No. 2006-501798 mentions a method of generating power at a constant current or a constant voltage in a fuel cell, controlling an operation point for power generation and charging a secondary battery with the power.
According to the art of Published Japanese Translation for PCT International Publication No. 2006-501798, however, when the fuel cell stack including the several series-connected fuel cells generates power at a constant current, the characteristic dispersion of each fuel cell disperses the output voltage of each fuel cell, though the generated current of each fuel cell is the same. Particularly, it is known that the lower the fuel stoichiometric ratio becomes to enhance the fuel utilization efficiency, the more easily the output voltage of each fuel cell disperses. Besides, if the cathode of a fuel cell produces flooding, the output voltage of the fuel cell may suddenly drop, thereby causing a polarity reversal to degrade the fuel cell.
On the other hand, in the art of Japanese Patent Laid-Open Publication No. 2006-73379, if the flow rate of a fuel supplied to each cell varies according to the output voltage of each fuel cell for the purpose of preventing the output voltage of each fuel cell from dispersing, then the fuel supply cannot be kept constant. The fuel-supply variation changes the fuel stoichiometric ratio, thereby making it hard to control the fuel stoichiometric ratio constant.
Accordingly, the arts of Published Japanese Translation for PCT International Publication No. 2006-501798 and Japanese Patent Laid-Open Publication No. 2006-73379 have the problem of making harder in reducing the dispersion of the output voltage of each fuel cell in the fuel cell stack including the several series-connected fuel cells and simultaneously in generating power while keeping a fuel stoichiometric ratio constant.